Britain’s battered high streets will take another pounding this week with retail experts predicting that the Black Friday phenomenon will push even more shoppers online and slash footfall during the crucial pre-Christmas sales period.

The Springboard group, which tracks shopper numbers at 4,500 locations across the UK, said it expects footfall will drop by 3.7% compared with Black Friday 2017, and by 2.7% over the weekend as a whole.

Black Friday, a day of pre-Christmas discounting borrowed from the US, is officially on 23 November, but Amazon has already begun its sales campaign. Along with other internet-based retailers, it has helped to turn Black Friday into a near fortnight-long £10bn shopping extravaganza, largely online.

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There are also signs that shoppers may be reaching exhaustion over Black Friday, partly because of squeezed incomes, but also because of a growing belief that the “bargains” offered may not be as attractive as they look.

Springboard anticipates that in the Saturday and Sunday after Black Friday, sales will fall by 5% compared with last year.

“Pressure on spending and reduced disposable income will be further compounded by the fact that Black Friday is taking place a week earlier this year, meaning that many consumers will not be paid until the week after, possibly forcing them to delay spending or rely on credit, which is already at its highest level since 2007,” said Springboard.

Shoppers are now accustomed to heavy discounting “almost continuously throughout the year, together with the widespread debate around whether Black Friday discounts are discounts at all”, it added.

Nine out of 10 Black Friday products are cheaper at other times of the year, according to separate research by price comparison site idealo.co.uk. It analysed prices of 21,047 products over a 12-month period and found that only laptops were offered at genuine bargain prices.

Smartphones are the most popular purchase on Black Friday, said Idealo, but they were on average £72 cheaper on 5 August. One of this year’s most popular new category of consumer electronics, drones, were typically £149 cheaper at the end of September.

Katy Phillips of Idealo said: “Many shoppers naturally believe the hype surrounding Black Friday and are seduced by the big-ticket deals. However, our advice is to stay savvy all year round.”

Some retailers now regard Black Friday as counter-productive, forcing them into discounting at a time when they traditionally made most of their annual profits.

Diane Wehrle of Springboard, said: “Whatever happens on Black Friday, our data over the past few years has established that it brings Christmas spending forward. This creates a magnet of spending activity at the beginning the peak trading period which then suppresses spending until the final week before Christmas.”